Overflow Ice: Dangerous and Vital to Arctic Rivers

An overflow ice sheet along the Aichilik River in the Arctic National Wildlife Refuge.

I was paddling down the upper Aichilik River in a small packraft. The Aichilik is a small, little-traveled river that flows north out of the eastern Brooks Range and across the coastal plain of the Arctic National Wildlife Refuge. It was mid-June, and the river, normally clear and blue, was running high and murky with snowmelt.

My two clients were new to paddling and the swift current was keeping everyone on their toes. As we floated, I shouted out instructions, urging my companions right or left across the current to avoid rocks, or clumps of uprooted willows that blocked the route downstream. In an area of tricky tight bends, I looked ahead and saw something that raises the adrenaline of arctic paddlers: ice.

Paddling beneath a sheet of aufeis in an ice canyon on the Aichilik River.

Overflow ice or aufeis, is a phenomenon of the arctic and other very cold environments. It works a little like this: During the winter, shallow streams and rivers freeze up from the surface to the bottom, but ground water and springs underneath continue to flow. That water has to go somewhere, so it spills out around the edges of the ice, or works its way up through cracks to the surface. There it freezes. As water continues to flow, more and more builds up, layer upon layer. In some areas, that ice can grow to glacial proportions, covering entire river valleys in ice, for depths of up to 10 meters, and extending for miles.

Freshly formed aufeis, or overflow ice, on the Koyukuk River in winter.

For paddlers of arctic rivers early in the summer, these areas of aufeis can be perilous. As spring run off cuts through the sheets, “ice canyons” form. In these blue, dripping canyons of ice, there are often few places to get out of the current. As the river and warm summer air erodes the ice, it collapses, at times dropping house-sized blocks of ice into water below. Scariest of all is when the rivers cut under the ice, a deadly formula for boats trying to make their way downstream.

When, paddling the Aichilik, I saw the blue ice coming up, I quickly pulled over, and motioned for my clients to do the same.

Scouting ice canyons is absolutely mandatory before trying to paddle through. So, I walked downstream, and made my way up onto the ice sheet scanning ahead with my binoculars. I could see the river wind through the ice, open water all the way. In this case, the passage was safe, and we paddled on, the blue cliffs looming overhead.

That was the first of several scouts I was forced to make that day, some of which required walking for half a mile or more along the ice to insure that there was a safe route through. Each time, we were able to pass safely through without having to make a troublesome portage. Still, when we reached the end of the ice, I breathed a sigh of relief.

Camp on the Canning River with a field of aufeis in the background. (Late June)

I’ve mostly seen overflow ice as a troublesome, and at times dangerous, part of recreating in Alaska. During winter, areas of overflow can have deep open water even in frigid temperatures, while in the summer, paddling through ice canyons can be terrifying. And other than noticing how ice sheets can change the vegetation that surrounds them, I’d never really considered the roll they play in river ecosystems.

Just north of the Brooks Range along the Katakturuk River, in late August, I found this patch of still-green willows and tundra in an area where overflow ice had recently melted out. Despite the rest of the region covered in autumn colors this patch still looked like spring.

On April 9, a paper was published by Tamlin Pavelsky and Jay Zarnetske in Geophysical Research Letters, a publication of the American Geophysical Union. The paper outlines how aufeis sheets in arctic Alaska are no persisting the way they once were. In the face of warmer summer temperatures, they are melting out far more quickly than they once did. And that melting may have consequences for Arctic river ecosystems and, as I think about it, for paddlers of arctic rivers as well.

The Brooks Range and the arctic are essentially deserts. The coastal plain looks from the air like a very wet place, but it is soggy only because the frozen, permafrost soils do not allow water to penetrate. So what precipitation does fall as winter snow or summer rains simply stays on the surface. But there really isn’t very much of that precipitation.

When I say the Arctic is a desert, that isn’t hyperbole. The coastal plain of the Arctic National Wildlife Refuge receives right around 10inches of precipitation a year and a desert is widely defined as areas that get 10 inches or less.

On backpacking trips, ice sheets can make for a rare area of good walking.

In a dry environment like that, the role of these ice sheets may be profound for the rivers. The sheets of overflow ice hold millions of gallons of frozen water which melts out slowly over the course of the summer, providing a steady source of water to the rivers. Without that ice, many of the rivers could be reduced to a trickle by mid-July.

Crossing an ice sheet in early June along the Kongakut River.

Grayling and Dolly Varden are two abundant fish in north slope rivers. The Grayling are residents while the Dollies are anadromous (running up the rivers to spawn but returning to the ocean in winter), but both depend on sufficient flows of water in the river.

And the rivers are important for more than fish. The entire aquatic ecosystem of fresh-water invertebrates to the shorebirds that feed on them, also depends on reliable water.

Melting aufeis on the Canning River, June.

In an earlier article here, I wrote about glaciers and shorebirds (I also just had a related piece published in the British conservation magazine Resurgence). But the presence of absence of aufeis hadn’t occurred to me as major part of the arctic river ecosystem. As glaciers disappear in the Brooks Range, which they will in the next 50 years or so, many rivers will depend even more upon the steady output of water by these ice sheets. If they too are disappearing, well, that ain’t good.

It ain’t good for the fish, and it ain’t good for assuring there is sufficient water to paddle my boats.

On the positive side of this story, I did learn something about overflow ice I’d never really considered: its role on the aquatic environment. This summer, as I’m forced to scout an ice sheet for a safe route through, at least I’ll have something to ponder other than worrying that the river will disappear beneath the ice.

Check out the original article in Geophysical Research Letters HERE.